In modern current mode controlled power supply converters, turn-off is determined by comparison of a feedback or reference signal, which is typically fed back from the secondary side to the primary side of a power converter for an output voltage regulation, and a current sensing signal or a power signal obtained from the power supply converter. FIG. 4 shows a possible realization of a circuitry suitable for determining the turn-off of such an external device. The circuitry of FIG. 4 is implemented on a control IC 800 (IC=integrated circuit) with an input 800a for the power signal or the current sensing signal Vcs, where cs stands for current sensing, an input 800b for the feedback signal, an input 800c for a ground potential, an output 800d for a switching signal dedicated for switching the power supply converter as described in more detail below and an input 800e for the power supply voltage Vcc of the control IC 800. In other words, the feedback voltage is typically fed back from the secondary side to the primary side of a power converter for output voltage regulation. Usually the output voltage is sensed by a voltage divider and then coupled via an optocoupler to the primary side of the power converter, which is often realized via an input of the control IC 800 in case an electrical isolation between the primary side and the secondary side is needed, but which is not shown in FIG. 4. Otherwise, the optocoupler is obsolete and does not have to be implemented and used.
Furthermore, FIG. 4 shows a transformer of a power supply converter 810, a power switch 820 with terminals 820a and 820b and a control input 820c for the switching signal, as well as a sensing resistor 830. The output 800d of IC 800 is connected to the control input 820c of power switch 820, and the input 800a of IC 800 is connected to a circuit node connecting terminal 820b of power switch 820 and resistor 830. Internally, control IC 800 comprises a surveillance means 840, and a switching means 850, which are connected in series between input 800a and output 800d. The surveillance means 840 is dedicated to check whether the voltage or power signal at input 800a assumes an undesired state indicative of a turn-off of the power supply converter and comprises an amplifying circuit 875, which, in turn, comprises an operational amplifier 880, and two resistors 890 and 900. The surveillance means 840 comprises also a comparator 910 and a flip-flop 920 with a reset input R for resetting the flip-flop, a set input S for setting the flip-flop and an output Q for outputting the current state of the flip-flop 920. The switching means 850 comprises a gate driver or driving means 860, as well as a protection means 870. The driving means 860 is dedicated to control the power switch 820 responsive to the control signal output by the surveillance means 840, while the protection unit is able to disable the gate driver 860.
The output of the power supply converter 810 is connected to the input 820a of the power switch 820, and the output 820b of the switch 820 is connected to both the sensing resistor 830 with the resistance Rsense and the input 800a of the control IC 800. Furthermore, the sensing resistor 830 is connected to the ground potential. Inside the controller IC 800, input 800a is connected to an input of the surveillance means 840 and to a non-inverting input of the operational amplifier 880, respectively. An output of the operational amplifier 880 is connected to a non-inverting input of the comparator 910, as well as via the resistor 890 to an inverting input of the operational amplifier 880. Further, the inverting input of the operational amplifier 880 is connected to an internal off-set voltage potential Vos via resistor 900, the operational amplifier 880 and the resistors 890, 900 thus forming a non-inverting amplifying circuit 875 providing an amplified signal V1derived from the power signal at the output of the operational amplifier 880 with the potential Vos being set to adjust the amplification behavior of the amplifying circuit 875 to a desired behavior. An inverting input of the comparator 910 is connected to the input 800b of the control IC 800. An output of the comparator 910 is connected to input R of flip-flop 920, while input S is connected to some activation signal to be described in more detail below and created externally with respect to the components of the IC 800 shown in FIG. 4 but, for example, internally with respect to the IC 800. The output Q of the flip-flop 920 represents the output of the surveillance means 840, where the control signal is output and is fed to an input of the switching means 850 and an input of the driving means 860, respectively. An output of the driving means 860 represents the output of the switching means 850 and is connected the output 800d of the control IC 800. Inside the switching means 850 the protection means 870 is connected to a control input of the driving means 860 via a signal line in order to influence the operation of the driving means 860 by a fault signal as will be described in more detail below.
After having described the design of the circuitry of FIG. 4, the operation of this design is described with respect to both, FIG. 4 and FIG. 5, wherein FIG. 5 shows a comparison of a time evolution or waveform of an exemplary amplified power signal V1 output by the amplifying circuit 875 and of a corresponding control signal Vc output by the surveillance means 840. The time is plotted against the x-axis in an aligned manner for the two vertically arranged graphs, and the voltages V1and Vcagainst the corresponding y-axes. As it will turn out from the following discussion, the circuit of FIG. 4 shows a part of a modern current mode controlled power supply converter or at least a part thereof, which turn-off is determined by a comparison of the feedback signal and the current sensing signal or the amplified power signal, respectively and IC 800 represents a system for the turn-off determination of a current mode controller. Exemplary waveforms occurring in IC 800 with regard to switching signal generation are shown in FIG. 5.
To monitor the state of the power supply converter or power supply 810, if turned on, the current created by the power supply or power supply converter 810 is fed through a sensing resistor or shunt resistor 830. The resulting voltage drop Vcs across the sensing resistor is fed as the power or current sensing signal to the control IC 800 and, in particular, to the input 800a of the control IC 800. The operational amplifier 880, along with the two resistors 890 and 900, amplifies the power signal and provides the comparator 910 with the amplified voltage signal V1. If this amplified voltage signal V1is larger than the feedback signal VFB, which is applied to the input 800b of the control IC 800, the comparator 910 outputs a signal, which is fed to the input R of the flip-flop, resulting in a reset of flip-flop 920. As a result, the output Q of the flip-flop or storage means 920 is erased or becomes LO, i.e. the control signal Vc becomes VLO. The driving means 860, which receives the control signal Vc, drives the gate of the power switch, as long as the fault signal provided by the protection means 870 is LO, depending on the control signal Vcnd drives the power switch 820 to be opened independent of the control signal Vc, if the fault signal is HI, for example. Of course circuit 800 could be alternatively designed such that the above mentioned logical signal values switch from LO to HI and vice versa. Thus, as a result of the control signal Vc getting LO, the switching signal provided at the output of the switching means 850 and, as a consequence, at the output 800d of the control IC 800 assumes a value resulting in the power switch 820 opening. To close the power switch 820 again, an activation signal is applied to the set input S of the flip-flop 920 by an internal or external means. The activation signal can be created by an element outside the circuit shown in FIG. 4, but which can be part of the control IC 800. Providing the set input S of the flip-flop 920 with the activation signal yields a change of the control signal provided to the switching means 850 and hence to the gate driver 860. The gate driver 860 will then create the switching signal, which is applied to the control input 820c of the power switch 820 resulting in closing the power switch 820 and hence a turn-on of the power supply converter 810 and a magnetization of the power transformer 810, respectively, unless the protection means 870 provides the gate driver 860 with a fault signal such that the gate driver 860 ignores the control signal and provides the power switch 820 with a switching signal yielding an open state of the power switch 820.
According to the example of FIG. 5 showing the time evolution of the amplified power signal V1and the control signal Vc, at the time t=t1 and t=t3 the storage means 920 is activated by an activation signal at the input S, which results in increasing the control signal Vc provided at the output Q from a voltage level VLO to a voltage level VHI. At the two moments t=t2 and t=t4, as illustrated in FIG. 5, the amplified power signal V1, becomes larger than the feedback voltage VFB, so that the comparator 910 provides the flip-flop 920 at its input R with a HI signal, so that the control signal Vc changes from a voltage level VHI to a voltage level VLO. As a result, the gate driver or driving means 860 provides the power switch 820 with a switching signal which results in switching off the power switch 820. In other words, in the current mode converter system shown in FIG. 4, the primary current is sensed by the shunt resistor or sensing resistor 830, wherein the voltage across the shunt resistor or sensing resistor 830 is amplified inside the control IC 800 depending on the off-set voltage Vos applied, and the turn-on is triggered by the internal or external activation signal at the set input S of the flip-flop 920. The output voltage V1 of the operational amplifier 880 is compared with the feedback voltage VFB by the comparator 910 for the switching-off signal or reset signal generation. As shown in FIG. 5, once the voltage V1 becomes larger than the feedback voltage VFB, the PWM flip-flop 920, which stands for pulse-width modulator, is turned over, and the power switch 820 is switched off.
The control IC 800 described above can be used to detect undesired states of operation of the power supply converter 810, wherein the states may be characterized by the power signal or, more precisely, by the signal derived from the power signal, i.e. the amplified power signal V1, exceeding the feedback voltage VFB. In other words, the voltage across the shunt resistor or sensing resistor 830 Vcs is compared with a certain reference voltage VFB for short-winding protection. Once the voltage exceeds the reference voltage, the protection circuit or the surveillance means 840 shuts down the gate driver or driving means 860, and the power switch 820 is then switched off.
However, this protection cannot provide full protection in some failure situations or in the case of an error related to the input 800a conveying the power signal of the control IC 800 or to the related pin. As shown above, the power signal or the voltage Vcs is one of the pieces of key information for turn-off determination. An error with respect to this pin can lead to drastic consequences. Apart from damage to or destruction of the control IC 800 or the power switch 820 or the power supply or the power supply converter 810, which can result in direct or indirect losses of money for repairs or replacement, also the health or life of human beings may be endangered.
Examples for errors with respect to input 800a of the control IC 800 comprise an open circuit at the pin related to input 800a, an interruption of one of the lines connected to the terminals of the sensing resistor 830 or a short circuit of the pin related to input 800a to ground. In other words, examples are open circuits of the pin Vcs to external, open shunt resistor or short circuit of this pin to the system ground. For instance, if the connection between the input 800a of the control IC 800 and the output of the switch 820b is broken, the voltage level of the power signal Vcs floats in an undetermined way as input 800a is directly connected to the non-inverting input 880a of the operational amplifier 880 of the surveillance means 840 due to the high input impedance of operational amplifiers in general. In this case, the power signal measured by the control IC 800 is no longer related to the primary current of the power transformer 810 in any way. This may lead to an unpredictable switching on and off of the power switch 820 and may also lead to operating the power supply or power supply converter outside of its specifications. In other words, once pin 800a is open circuit to external, the voltage at this pin is then floating since this pin is connected to the high Ohmic input of operation amplifiers or operators internally. As a result, the turn-off of the main power switch is not in relation to the real primary current, and it may result in unpredictable situations for the converter system, not regulated output voltage or even damage of the power converter system.
In the case where the pin related to input 800a of the control IC 800 is short-circuited to ground, the surveillance means 840 is not able to turn off the switch 820 at all, as the power signal Vcs measured by the control IC 800 and the surveillance means 840 is always smaller than the feedback signal VFB provided to the control IC 800 and the surveillance means 840, so that the comparator 910 is not able to provide the storage means 920 with a HI as reset signal. In other words, in the case where the pin related to input 800a is short-circuited to ground, the main power switch 820 will never be switched off, which leads to saturation of the transformer 810, over-current of the main power switch 820 and possible permanent damage to the power converter or power supply. In other words, both an open circuit and a short circuit to ground of this pin, may destroy the whole converter system.
If, as a third example, the line between switch output 820b and the sensing resistor 830 is broken or the connection between the sensing resistor and the ground is broken, such that the primary current has to pass input 800a of the control IC 800, a very high voltage can appear at the operational amplifier 880 due to the high input impedance of operational amplifiers in general, such that the control IC 800 can be destroyed yielding an undefined state of the switch 820, which may result in damage to or the destruction of the power supply converter.
From the above follows, that some protection scheme should be applied to the pin related to input 800a to prevent such consequences from happening due to failures of non-proper connection of this pin. So far, no protection against these failures is in the current situation available in existing control ICs.